An open letter signed by more than 100 imams from across major theological backgrounds and cultural groups has urged British Muslim communities "to continue the generous and tireless effort to support all of those affected by the crisis in Syria and unfolding events in Iraq", but to do so "from the UK in a safe and responsible way".

The letter comes during Ramadan, but against a troubling backdrop of tensions between the Middle East and the West.

The situation has been underlined by US officials who have stepped up security precautions amid reports that two terror networks are working together on a bomb that could breach existing measures.

Concerns have also been raised about home-grown involvement in terrorism, after Britons appeared in a propaganda video for insurgent group Islamic State in Iraq and Greater Syria (Isis).

Among them was aspiring jihadi Aseel Muthana, who told the BBC that he was fighting in Syria and had no intention of returning to the UK.

His brother Nasser appeared with two other British men – 20-year-old Reyaad Khan, from Cardiff, and Abdul Raqib Amin, who grew up in Aberdeen – in an Isis propaganda video.

Earlier this week, it was revealed that a Londonderry man is believed to be fighting for rebels in Syria. He is understood to have told his mother he was going to Turkey on holiday, but instead he gained passage to Syria where he has joined a group fighting in the civil war.

The open letter read: "As the crisis in Syria and Iraq deepens, we the under-signed have come together as a unified voice to urge the British Muslim communities not to fall prey to any form of sectarian divisions or social discord.

"Ramadan, the month of mercy, teaches us the value of unity and perseverance and we urge the British Muslim communities to continue the generous and tireless efforts to support all of those affected by the crisis in Syria and unfolding events in Iraq, but to do so from the UK in a safe and responsible way."

In April, the Met issued a plea for people to come forward with information if they were concerned about them joining terrorist training camps in Syria.

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